The DC-to-RF conversion efficiency of a RF, microwave or millimeter-wave system is a key driving function for systems where prime power is limited. Applications requiring high efficiency transmitters range from Ka-band communication links for deep space probes to battery powered cellular telephones at 900 MHZ. While efficiency is clearly a key system driver for systems with limited prime power, it may also impact the system in more subtle ways. For example, inefficient power integrated circuits (IC's) in the transmitter can create power dissipation problems for the system and undue thermal stress on the system components including the IC's. That, in turn, may result in poor reliability and poor system performance, thereby dramatically impacting system cost.
While most MMIC designs focus only on the efficiency of the chip, in a system application, other sources of power loss are equally, if not more important to the overall system efficiency. Hence, to maximize system efficiency, it is important to examine how the chip is used in the system. For example, the avionics of a typical space-based system employs a "system bus" which is typically in the range of 20 to 30 volts. A power conditioning subsystem, consisting of a DC-to-DC converter and a series voltage regulator, provide the actual conditioned power to the transmitter IC's. In such a system, the converter has an efficiency ranging from 75 to 90 percent, depending on the output voltage. The regulator generally requires a DC voltage drop of at least 2 volts resulting in a further efficiency degradation of typically 75 percent (8 to 6 volts). Together, those two sources of inefficiency can reduce a respectable chip efficiency of, say, 30% down to 18% at the system level. Both the converter efficiency and the regulator efficiency can be improved by using a higher chip supply voltage. That is true because, in both cases, the source of the power loss is due to a fixed voltage and/or IR drop.
While increasing the chip supply voltage certainly improves the efficiency of the power conditioner, it unfortunately does not improve the efficiency of the active devices in the integrated circuit. That is due to the fact that maximum device power-added efficiency is realized for a voltage of typically 4 to 5 volts for Ka-band PHEMT's (Pseudomorphic High Electron Mobility Transistors). The traditional chip supply voltage of 6 to 7 volts represents a compromise between the device efficiency, output power and the power conditioner efficiency. While there are a few things that may be done with the device design and the GaAs doping profile to raise this voltage, they generally compromise the maximum efficiency and output power and only raise the terminal voltage marginally. Therefore, for high-efficiency power applications, a better method of raising the chip supply voltage is needed. Clearly, if a way could be found to raise the chip voltage without sacrificing the device efficiency, the overall system efficiency and reliability would benefit greatly. Further, if it were possible to raise the chip supply voltage high enough, it might even be possible to eliminate the DC-to-DC converter.
The present invention disclosure addresses the problem of DC-to-RF conversion efficiency of GaAs monolithic circuits (MMICS) used in RF, microwave and millimeter-wave communications systems.